A Characteristic Study on NIPAM Gel Dosimetry Using Optical-CT Scanner

This study investigated the dose characteristics of N-isopropylacrylamide (NIPAM) polymer gel dosimetry in intensity-modulated radiation therapy (IMRT). The NIPAM gel was composed of 5% gelatin, 5% NIPAM, 3% Bis, and 5 mM tetrakis (hydroxymethyl) phosphonium chloride (THPC). The gel was poured into a cylindrical acrylic phantom with a diameter of 10 cm, a height of 10, and a wall thickness of 3 mm. The gel phantom was irradiated with IMRT. The phantom energy was 6 MV and the dose rate was 250 MU. The NIPAM gel was scanned using an optical computed tomography (CT) scanning system. In terms 2of uniformity, the intra-dosimeter showed a consistent dose profile at different depths and a deviation of less than 1.8%. The scanning results showed a consistent dose distribution for each scanning experiment. The percentage isodose lines from the measured data agreed well with those from the treatment planning system (TPS) at 60% to 100% dose level region which is acceptable for clinical applications. Gamma index analysis was performed for representative gamma comparison between the TPS and the measurement results. The acceptance pass rate was calculated for various criteria. The pass rates were as high as 99.5% and 97.8% with 5%/5 mm and 4%/4 mm gamma acceptance criteria, respectively. The results indicate that a NIPAM polymer gel dosimeter can be used in conjunction with optical CT as a dose verification tool, especially for three-dimensional dose verification.